The vast majority of all organic polymeric materials undergo some mode of degradation when exposed to the high energy photons of ultraviolet light. The degradation manifests itself depending on the material in yellowing, discoloration, embrittlement and other loss of physical properties. Polycarbonate resin is no exception and it is, therefore, an object of this invention to provide an improved method of producing a polycarbonate resin article which is highly resistant to ultraviolet radiation degradation.
The use of certain compounds as ultraviolet radiation absorbers is well known in the art. Heretofore, these compounds have been incorporated into the polymer melt prior to forming the structure; coated on the surface of the polymeric structure; or impregnated into the surface of the structure by swelling the substrate by use of a solvent in which the ultraviolet light absorber is dissolved, or by embedding an ultraviolet light absorber in the substrate by melting both the absorber and the surface of the substrate, or by using aqueous solution of absorbers which are more soluble in the polycarbonate substrate than in the aqueous solution. Each of these prior art methods has certain disadvantages. The first method, incorporating the ultraviolet radiation absorbing compound in the melt, distributes the compound throughout the structure. This procedure is both uneconomical, as these compounds are usually quite expensive, and not completely successful. Much of the harmful radiation penetrates and deteriorates the structure before reaching the too well-distributed absorber. Also, the concentration of the absorber in the resin is limited by the degree of compatibility of said compound with the polycarbonate. In the second method, applying a coating of the ultraviolet radiation absorbing compound onto the surface of the article, difficulties are sometimes encountered in adhering the coatings to the surface of the structure. The coatings may not adhere well, or if they do adhere, they often cannot be applied without forming streaks. Furthermore, the coating, which usually contains a polymeric material, may also tend to be degraded by ultraviolet light and peel from the base structure. The various prior art surface impregnation processes also have certain disadvantages. Using a solvent to swell the substrate may, due to the solvent's or swelling agent's active nature, degrade the advantageous and desired properties of the polycarbonate article. Imbedding the compound in the surface of the substrate by melting both the compound and the substrate likewise may tend to degrade the properties of the polycarbonate resin. Using aqueous solutions containing an ultraviolet light absorbent compound which is more soluble in the polycarbonate than in water rather sharply limits the number and type of ultraviolet light absorbent compounds which can be used.
The instant invention provides an economical and effective method for protecting polycarbonate shaped articles from degradation by ultraviolet radiation by incorporating ultraviolet radiation absorbent compounds into the polycarbonate structure so that the compounds are concentrated uniformly at the surface of the polycarbonate structure.